Pulsed lasers make it possible to achieve high instantaneous powers for a very short duration of approximately several picoseconds (10−12 s) or several femtoseconds (10−15 s). In these lasers, an ultra-short laser pulse is generated in an oscillator before being amplified in amplifying mediums. The laser pulse initially produced, even with low energy, creates a high instantaneous power, since the energy of the pulse is delivered in an extremely short time.
To make it possible to increase the energy of the laser pulse without the very high instantaneous power creating nonlinear effects, it has been considered to time-stretch the pulse before it is amplified, then to recompress it after amplification. The instantaneous powers used in amplifying mediums can thus be decreased. This method, called “frequency-drift amplification” (frequently called CPA, for “Chirped Pulse Amplification”), makes it possible to increase the length of a pulse by a factor of approximately 103 to 105, then to recompress it so that it regains a duration close to its initial duration.
This CPA method, described in the article by D. Strickland and G. Mourou, “Compression of amplified chirped optical pulses” (Opt. Commun. 56, 219-221-1985), uses a spectral decomposition of the pulse, making it possible to impose a path with a length different from the various wavelengths to time-shift them.
FIG. 1 diagrammatically shows the amplification of the laser pulse using this chirped pulse amplification method.
An oscillator 1 emits a laser pulse 91, called input pulse, with a very short duration ΔT, for example 10 femtoseconds, and relatively low energy E, for example approximately several nanojoules. This input pulse 91 passes through a stretcher 2 that distributes its various spectral components over time as a function of their wavelength.
Several methods can be used to produce the stretcher 2.
One commonly used stretcher 2 implements diffraction gratings reflecting the incident light rays with different orientation depending on the wavelength. The structure of such a stretcher is in particular described in the article by O. E. Martinez, “3000 times grating compressor with positive group velocity dispersion: application to fiber compensation in 1.3-1.6 μm region” (IEEE Journal of quantum Electronics, Vol. qe-23, p. 59, 1987).
The different spectral components forming the input pulse 91 do not travel the same path in the stretcher 2. Depending on the construction of said stretcher 2, the components with shorter wavelengths must travel a longer path, or on the contrary a shorter path than the components with a larger wavelength. This difference in the length of the path causes a time-shift of the spectral components as a function of their wavelength in the pulse 92, which is called a stretched pulse.
This stretched pulse 92 consequently has a greater duration than the duration ΔT of the input pulse 91, which may for example be approximately 105 ΔT. This greater duration causes a very significant decrease in the instantaneous power of this pulse 92 relative to that of the input pulse 91, which allows it to be amplified under better conditions.
Another method that can be used to stretch laser pulses is the propagation of those pulses in optical fibers over long distances. The group speed dispersion of the spectral components of the pulse in the material at the core of the fiber makes it possible to obtain the desired time-elongation.
Still another known stretching method consists of a Bragg diffraction grating made from a photosensitive material, whereof the pitch is not constant with respect to the thickness. The different spectral components of the laser pulse are then reflected at different depths, which creates a delay for certain spectral components relative to others and thereby stretches the pulse. Such a method is in particular described in the article by Vadim Smirnov, Emilie Flecher, Leonid Glebov, Kai-Hsiu Liao, and Almantas Galvanauskas, “Chirped bulk Bragg gratings in PTR glass for ultrashort pulse stretching and compression” (Proceedings of Solid State and Diode Lasers Technical Review. Los Angeles 2005, SS2-1.).
The stretched pulse 92 leaving the stretcher 2 is then amplified using traditional amplifying mediums, which increase its power. As an example, three amplifying mediums are shown in FIG. 1.
The first amplifying medium 3, called “high gain amplifier,” increases the power of the stretched pulse 92 until giving it an energy of approximately 106 times the energy E of the incident pulse 91, for example several millijoules. The second amplifying medium 4 and the third amplifying medium 5 also increase the power of the laser pulse such that the amplified stretched pulse 93 has an energy of approximately 1010 times the energy E of the input pulse 91, for example 25 joules.
Although the pulse has a relatively significant energy, its duration is relatively long, which means that its peak power is low enough to avoid non-linear effects in the amplifying mediums 3, 4 and 5.
The amplifying medium used is most often a stimulated emission amplifying medium, for example such as a titanium-doped sapphire crystal. According to one possible alternative, the amplification of the laser pulse can be done using the method typically called “Optical Parametric Chirped Pulse Amplification,” which combines laser pulse parametric amplification with the chirped pulse amplification technique. This amplification method is in particular described in the article by A. Dubietis et al. “Powerful femtosecond pulse generation by chirped and stretched pulse parametric amplification in DBO Crystal” (Opt. Commun. 88, 433 (1992)).
Amplifiers using stimulated emission amplification or parametric amplification are indifferently designated as “amplifying mediums” in the following of this patent application.
The return of the pulse to a very short duration, close to the duration ΔT of the input pulse, is done in an optical device called a compressor 6, comprising four diffraction gratings 61, 62, 63 and 64 reflecting the incident light rays with a different orientation depending on the wavelength.
Thus, a first grating 61 spectrally disperses the stretched pulse 93. As an illustration, the three rays 911, 912 and 913, corresponding to two extreme wavelengths of the pulse 910 and one middle wavelength, are shown in FIG. 1.
The second grating 62 returns the spectral components in parallel, in particular 911, 912 and 913, making up the laser pulse, which are thus spatially spread out. The third grating 63 makes it possible to bring these four spectral components together in a same point of the fourth grating 64, which returns all of the spectral components, in particular 911, 912 and 913, in the same direction, to form a new laser pulse 94.
The different spectral components, in particular 911, 912 and 913, forming the input pulse 91 do not travel the same path in the compressor 6. More specifically, the compressor 6 is built such that the spectral components that have a longer path in the stretcher 2 have a shorter path in the compressor 6. This length difference in the path causes a time shift of the spectral components as a function of their wavelength, opposite to the shift generated by the stretcher 2.
Thus, the spectral components that were time-delayed in the pulse 92 or 93 make up their delay, such that all of the spectral components are temporally gathered in an output pulse 94 having a duration similar to the duration ΔT of the input pulse 91, for example 20 femtoseconds, and a very high peak power, for example approximately 1014 W.
The chirped pulse amplification technique therefore makes it possible to produce laser pulses with a very high instantaneous power.
In the devices typically used for chirped pulse laser amplification, the adjustment of the desired duration of the final amplified pulse is done by moving the components of the compressor, so as to modify the characteristics of the compression. More specifically, it is necessary to modify the angle of the diffraction gratings and their relative position.
For high-power lasers, the compressor 6 having to bear very high energy levels, the diffraction gratings making it up are large and must be placed in a vacuum chamber. Their handling to adjust the duration of the pulse or to adjust the time thereof, which requires modifications to the orientation of the gratings and the distances separating them, is therefore particularly awkward.
The precise adjustment of the duration of the final pulse is particularly problematic. In fact, this adjustment must be done as a function of experimental conditions, and may be modified over the course of an experiment or a series of experiments. To perform this adjustment without opening the vacuum chamber, certain chirped pulse amplification devices comprise diffraction gratings whereof the movements are motorized. This technical solution nevertheless creates significant costs.
Furthermore, the chirped pulse amplification devices generate time aberrations in the laser pulse, and in particular time phase residuals. These aberrations, which primarily appear in the shortest pulses (having a duration before stretching of less than 30 fs) having a greater spectral width, make it impossible to compress the pulse to the shortest duration that could theoretically be obtained.
In certain chirped pulse amplification devices, programmable systems for modifying the phase and amplitude may be installed to perform the time adjustment and correct the time aberrations of the pulse. These systems can only, however, withstand relatively low-energy pulses, and what is more, for some, create even more significant energy losses when the correction is significant. Moreover, the cost of these programmable phase and amplitude modification systems is very high.